Prior art solvent refined coal (SRC) technology is exemplified by U.S. Pat. No. 3,341,447. In the preferred embodiments of the invention the SRC process employed is known as SRC-I. In accordance with the present invention, solvent deashed SRC may be blended with process derived solvent and mildly hydrotreated to produce a low-sulfur, low-ash boiler fuel.
The basic SRC-I process requires coal-handling, pulverization of the coal in the range of 10 to 300 mesh and mixture, along with gaseous hydrogen, in a solvent that is hydroaromatic in nature and is capable of donating hydrogen to the coal particles. Solvent-to-coal ratios of between 1.5 parts and 3 parts of solvent to 1 part of coal are typically used. This mixture of solvent and coal plus hydrogen is pressurized to between about 500 to 3000 psig (35.1 to 210.8 kg/cm.sup.2 gauge) and sent through a preheater, which heats the mixture to roughly 805.degree. F. (429.4.degree. C.). Some of the coal dissolves in the preheater.
The three-phase mixture then goes into a reactor, known as the dissolver. In the dissolver, additional dissolution, hydrogenation and desulfurization of coal occurs. The pressure of the dissolver is held close to 2000 psig (140.5 kg/cm.sup.2 gauge), with a residence time conventionally ranging from two minutes up to two hours. During this time, coal is dissolved and hydrocracked, as bonds are broken and sulfur reacts with hydrogen to form H.sub.2 S. After the reaction, the three-phase effluent mixture is flashed and cooled, with gases being removed, including hydrogen, which may be recycled after purification. In one embodiment of the SRC-I process, the coal liquids and undissolved coal plus ash, after distillation to remove the 850.degree. F.- fraction, go through a solvent deashing process, which is a liquid/solid separation step that separates ash and undissolved coal from the SRC and a heavy fraction of process derived solvent. The hydrogen-donating process-derived solvent fraction that was removed in the vacuum distillation step, e.g., the fraction boiling between about 400.degree. F. and 850.degree. F. is recycled to the front-end of the process. The 850.degree. F.+ (429.4.degree. C.+) boiling material may then be solidified as product; i.e., solvent refined coal which is also referred to by the acronym "SRC". The typical sulfur concentration in SRC is between about 0.7 and 1.5 weight percent, depending on the feed coal and the operating conditions. Total coal conversion on a moisture and ash-free basis is between 80% and 98%. The process has a hydrogen consumption of between about 1% to 31/2% based on MAF (Moisture Ash Free) coal.
U.S. Pat. No. 4,374,015 discloses an SRC-I type process which employs a solvent deashing process of the type utilized in accordance with the invention. While a portion of the heavy soluble deashed SRC material (i.e., FIG. 1, line 68 of U.S. Pat. No. 4,374,015) is recycled to the front-end of the coal liquefaction process with optional hydrotreating for hydrogen enrichment in order to provide a portion of the solvent necessary to liquefy coal feed, the SRC material taken as product is not further treated to reduce sulfur or other heteroatoms.
In a variation of the SRC-I process, identified as SRC-II, a portion of the product slurry is recycled as a "solvent" for the coal rather than using an all-distillate liquid as in the original SRC-I process. In addition to dissolving the coal, the SRC-II process also converts much of the dissolved coal to distillate liquid and gaseous products. The major liquid product is separated in a vacuum distillation unit, the undistilled 850.degree. F.+ product which is termed "vacuum residue", is fed to a gasifier for hydrogen production. In theory, the quantity of organic material remaining in the vacuum residue is controlled so that it is just sufficient to produce the hydrogen required for the process. Thus, conversion of a large part of the dissolved coal to liquids makes it possible to eliminate the liquid/solid separation step required in the SRC-I process. Hydrogen consumption is close to 5% in this SRC-II process.
Depending, in part, on the composition of the coal employed, the SRC produced from the SRC-I process may be too high in sulfur content, so that the SO.sub.2 content of flue gases resulting from burning of the solid SRC product would not meet environmental standards. While the product produced by the modified SRC-II process has a lower sulfur content than that produced under the SCR-I, it is inherently more expensive than the SRC-I process, due to substantially larger hydrogen requirements. Also, the products of the SRC-II process are largely liquid rather than solid.
A number of coal conversion processes which involve a thermal liquefaction step, followed by a catalytic hydroprocessing step, have been developed.
The Consol Synthetic Fuel (CSF) Process has been demonstrated in a 20 ton/day pilot plant at Cresap, W. Va. In this process, preheated coal is slurried in a recycle solvent and reacted in a stirred vessel at low pressure (.about.500 psig, or 35.1 kg/cm.sup.2 gauge) and at a temperature of 750.degree. to 800.degree. F. (398.9.degree. to 426.7.degree. C.). No hydrogen is added to the coal/solvent mixture prior to the reactor. The liquid products from the dissolver are passed through hydroclones to remove ash and unreacted coal. The solids stream from the hydroclone is coked at low temperature (ca. 900.degree. F., or 482.2.degree. C.) to produce a liquid stream and a char, which is gasified to produce hydrogen. The deashed products (vapor and liquid) from the dissolver, plus the liquid and gaseous products from the coking step, are sent to a separation section, where light gases, light distillate and recycle solvent are recovered. The bottoms product and the tars from the coking step are then hydrotreated. The hydrotreating conditions are relatively severe, such that most of the feed is converted to gas, naphtha and middle distillate. The small amount of residue that remains after this hydrotreating is typically used as a plant fuel. Because the hydrotreating step is so severe the product is essentially a distillate fuel.
The H-Coal Process, developed by Hydrocarbon Research, Inc., is generally regarded as a purely catalytic coal liquefaction process. However, it is likely that some small amount of "thermal" liquefaction takes place in the preheater upstream of the catalytic reactor. In this process, a slurry of coal, recycle solvent and hydrogen is passed through a fired preheater and is then fed to an ebullated-bed reactor which operates at about 850.degree. F. (454.4.degree. C.) and 2500 psig (175.7 kg/cm.sup.2 gauge). In the reactor, the coal is liquefied and hydrocracked. Two modes of operation are possible, a "syncrude" mode and a "boiler fuel" mode. In the "syncrude" mode, severe hydrocracking takes place and the product is mostly distillate material. In the "boiler fuel" mode, the hydrocracking severity is less, and a substantial quantity of solid product is produced. However, this product has relatively high sulfur concentration, about 1 weight percent. Neither mode produces the desired product in a substantial yield, and the residence time for the thermal step is probably too low to accomplish much desulfurization or breakdown of coke precursors.
The Synthoil Process, developed by the U.S. Bureau of Mines, is similar to the H-Coal Process, except that hydroprocessing is carried out in a fixed-bed reactor, rather than an ebullated-bed reactor. As with H-Coal, some coal conversion probably takes place as the coal/recycle solvent/hydrogen slurry is passed through the preheater prior to the reactor. The objective of the Synthoil Process is to convert coal essentially completely to distillate products. Therefore, the hydroprocessing reactor is operated at a high severity.
The Exxon Doner Solvent (EDS) Process also involves both a thermal liquefaction step and a catalytic hydroprocessing step. The thermal liquefaction step is carried out at conditions very similar to those used for the SRC process. However, the recycle solvent is specially selected and catalytically hydrogenated in order to substantially increase its hydrogen-donor capacity. In the thermal liquefaction step, coal is converted essentially to liquid products, with only a small amount of heavy residue remaining. The effluent from the thermal step is distilled. One of the cuts is the recycle solvent, which is catalytically hydrotreated to increase its hydrogen-donor capacity, as noted above. The bottoms from the distillation step are coked or gasified to produce the hydrogen required for catalytic hydrotreating of recycle solvent and for the thermal liquefaction step.
However, this process differs from the present invention in that (1) no residual (initial boiling point &gt;850.degree. F., or 454.4.degree. C.) solid fuel is produced; heavy materials (initial boiling point &gt;850.degree. F., or 454.4' C.) are coked or gasified rather than being hydroprocessed into a low-sulfur boiler fuel, and (2) only middle distillate (400.degree. to 850.degree. F., or 204.4.degree. to 454.4.degree. C.) is hydrotreated; no residue is hydrotreated. p U.S. Pat. Nos. 3,594,303, 4,123,347 and 4,152,244 are illustrative of the coal solvation patent art showing a thermal solvent refining step followed by a catalytic hydroprocessing step. These processes are directed, however, to maximizing the production of liquid hydrocarbons from coal and not the production of desulfurized and deashed solid boiler fuel as a primary product. U.S. Pat. Nos. 3,884,796 and 3,892,654, also show two stage systems, with the second stage comprising hydrogenation; however, the products are separated by distillation and filtration without further hydrogenation.
Among the objects of this invention are to provide a new and improved process for producing a boiler fuel from solvent refined coal having very low sulfur and ash content by a two-stage process with minimal hydrogen consumption having a first thermal solvent refining step and a second hydrotreating step, which produces desulfurization and denitrogenation without substantial reduction in asphaltene and preasphaltene levels or substantial coking.